1. Field of the Invention
The present invention relates to the papermaking arts. More specifically, the present invention is a papermaker's or dryer fabric for use on the dryer section of a paper machine, such as on a single-run dryer section.
2. Background of the Invention
During the papermaking process, a cellulosic fibrous web is formed by depositing a fibrous slurry, that is, an aqueous dispersion of cellulose fibers, onto a moving forming fabric in the forming section of a papermaking machine. A large amount of water is drained from the slurry through the forming fabric, leaving the cellulosic fibrous web on the surface of the forming fabric.
The newly formed cellulosic fibrous web proceeds from the forming section to a press section that includes a series of press nips. The cellulosic fibrous web passes through the press nips supported by a press fabric, or, as is often the case, between two such press fabrics. In the press nips, the cellulosic fibrous web is subjected to compressive forces which squeeze water therefrom, and which adhere the cellulosic fibers in the web to one another to turn the cellulosic fibrous web into a paper sheet. The water is accepted by the press fabric or fabrics and, ideally, does not return to the paper sheet.
The web, now a paper sheet, finally proceeds to a dryer section, which includes at least one series of rotatable dryer drums or cylinders, which are internally heated by steam. The newly formed paper sheet is sequentially directed in a serpentine path around each in the series of drums by one or more dryer fabrics, which hold the paper sheet closely against the surfaces of the drums. The heated drums reduce the water content of the paper sheet to a desirable level through evaporation.
It should be appreciated that forming, press and dryer fabrics all take the form of endless loops on the papermaking machine and function in the manner of conveyors. It should further be appreciated that paper manufacture is a continuous process which proceeds at considerable speeds. That is to say, the fibrous slurry is continuously deposited onto the forming fabric in the forming section, while a newly manufactured paper sheet is continuously wound onto rolls after it exits from the dryer section.
The instant invention relates primarily to a dryer fabric for use in the drying section of a papermaking machine. In a dryer section, the dryer cylinders may be arranged in a top and a bottom row or tier. Those in the bottom tier may be staggered relative to those in the top tier, rather than being in a strict vertical relationship. As the sheet proceeds through the dryer section, it may pass alternately between the top and bottom tiers, first passing around a dryer cylinder in one of the two tiers and then passing around a dryer cylinder in the other tier, and so on sequentially through the dryer section. As depicted in FIG. 1a, in dryer sections, the top 94 and bottom 96 tiers of dryer cylinders may each be clothed with a separate dryer fabric 99. In such a situation, paper sheet 98 being dried passes unsupported across the space, or “pocket”, between each dryer cylinder and the next dryer cylinder on the other tier.
In a single tier dryer section, a single row of cylinders along with a number of turning cylinders or rolls may be used. The turning rolls may be solid or vented. In a single-tier dryer section, such as that shown in FIG. 1b, a paper sheet 198 is transported by use of a single dryer fabric 199 which follows a serpentine path sequentially about dryer cylinders 200 in the top and bottom tiers.
In order to increase production rates and minimize disturbance to the sheet, single-run dryer sections are used to transport the sheet being dried at high speeds. In a single-run dryer section, a single dryer fabric follows a serpentine path sequentially about the dryer cylinders in the top and bottom tiers.
It will be appreciated that, in a single-run dryer section, the dryer fabric holds the paper sheet being dried directly against the dryer cylinders in one of the two tiers, typically the top tier, but also carries it around the dryer cylinders in the bottom tier. The fabric return run is above the top dryer cylinders. On the other hand, some single-run dryer sections have the opposite configuration in which the dryer fabric holds the paper sheet directly against the dryer cylinders in the bottom tier, but also carries it around the top cylinders. In this case, the fabric return run is below the bottom tier of cylinders. In either case, a compression wedge is formed by air carried along by the backside surface of the moving dryer fabric in the narrowing space where the moving dryer fabric approaches a dryer cylinder. The resulting increase in air pressure in the compression wedge causes air to flow outwardly through the dryer fabric. This air flow, in turn, forces the paper sheet away from the surface of the dryer fabric, a phenomenon known as “drop off”. “Drop off” can reduce the quality of the paper product being manufactured by causing edge cracks and can also cause sheet breaks, which reduces machine efficiency.
Many paper mills have addressed this problem by machining grooves into the dryer cylinders of the lower tier or by adding a vacuum source to those dryer cylinders. Although both are expensive, both of these expedients allow the air otherwise trapped in the compression wedge to be removed without passing through the dryer fabric. Furthermore, non-sheet marking and sheet contact with the dryer cylinder surface are requirements as well as good wear resistance and dimensional stability.
As previously discussed, it should be appreciated that the forming, press and dryer fabrics all take the form of endless loops on the paper machine and function in the manner of conveyors. A seam, such as a seam which may be used to close a fabric into endless form during installation on a papermaking machine, represents a discontinuity in the uniform structure of the fabric. The use of a seam, then, greatly increases the likelihood that the cellulosic fibrous web will be marked during the drying process.
For this reason, a seam is generally a critical part of a seamed fabric because uniform paper quality, low marking and excellent runnability of the fabric require a seam which is as similar as possible to the rest of the fabric in respect of properties such as thickness, structure, strength, permeability etc. Consequently, the seam region of any workable on-machine-seamable fabric must behave similar to the body of the fabric, and should have similar permeability to water vapor and to air as the rest of the fabric in order to prevent the periodic marking by the seam region of the paper product being manufactured.
Despite the considerable technical obstacles presented by these requirements, it remained highly desirable to develop on-machine-seamable fabric because of the comparative ease and safety with which such a fabric could be installed on a papermaking machine. Furthermore, in the dryer section, fabrics must have a seam to allow installation. Ultimately, these obstacles were overcome with the development of fabrics having seams formed by providing seaming loops on the crosswise edges of the two ends of the fabric. The seaming loops themselves are formed by the machine-direction (MD) yarns of the fabric. A seam is formed by bringing the two ends of the fabric together, by interdigitating the seaming loops at the two ends of the fabric, and by directing a so-called pin, or pintle, through the passage definerd by the interdigitated seaming loops to lock the two ends of the fabric together. Needless to say, it is much easier and far less time-consuming to install an on-machine-seamable fabric, than it is to install an endless fabric, on a papermaking machine.
One method of producing a fabric that can be joined on a papermaking machine with such a seam is to flat-weave the fabric. In this case, the warp yarns are the machine-direction (MD) yarns of the fabric. To form the seaming loops, the warp yarns at the ends of the fabric are turned back and woven back some distance into the fabric body in a direction parallel to the warp yarns.
In some cases, spiral seaming coils may be attached to the seaming loops at the ends of the fabric by, for example, interdigitating the individual turns of a spiral seaming coil with the seaming loops via a pin or another CD (cross machine direction) body yarn at each end of the fabric and by directing a pintle through the passage formed by the interdigitated yarns and seaming loops to join the spiral seaming coil to the end of the fabric. Then, the fabric may be joined into the form of an endless loop by interdigitating the individual turns of the seaming coils at each end of the fabric with one another, and by directing another pintle through the passage formed by the interdigitated seaming coils to join the two ends of the fabric to one another. As known to those skilled in the art, many varieties of industrial fabrics are designed to be closed into endless form during installation on some equipment.
Besides dryer fabrics, other industrial fabrics, such as corrugator belts, pulp-forming fabrics and sludge-dewatering belts are, or can be, seamed in similar fashion. In these fabrics, where the MD yarn is also the seam loop, it is well known that bending a yarn, especially a single monofilament, around a small radius to form a loop, stresses and weakens the yarn in the loop area. The whole seam is then weaker than the main fabric body in use. Since the seam loops are load bearing and are flexed repeatedly (and in some cases also compressed) during use, any machine upset can lead to premature seam failure and fabric removal.
An important aspect of seaming a fabric on a papermaking machine is that there be uniform tension across the fabric. If uniform tension is not achieved and one section of the fabric pulls more than another, then the fabric can bubble or ridge across the fabric width.
Another aspect of seaming a fabric is preventing damage to the fabric body. In order to avoid or minimize the chance of damage to the fabric during installation, non-uniform tension, weight and pressure must be avoided on the seam itself. A further aspect of seaming a fabric, especially very long ones is properly aligning the fabric body in the machine so the fabric guides true in the machine direction and does not oscillate or track to one side of the machine. If the fabric guides or tracks poorly it can make contact with the paper machine support frame and cause fabric damage.
Consequently, seaming a conventional dryer fabric on a papermaking machine is a difficult and tedious process. Therefore, a need exists for a dryer fabric that can be installed and seamed quickly and easily. In addition to being installed easily and quickly, the instant dryer fabric is both durable and provides a smooth sheet contacting surface in both the fabric body and the seam region, resulting in less marking and thus higher quality of a paper sheet being manufactured thereon.